Attempt to get the keys to the garrison (6)
I believe the answer is:
effort
'attempt' is the definition.
(synonyms)
'the keys to the garrison' is the wordplay.
'the keys' becomes 'ef' (I can't explain this - if you can you should give a lot more credence to this answer).
'to the garrison' becomes 'fort' ('fort' can be a synonym of 'garrison').
'ef'+'fort'='EFFORT'
'to get' acts as a link.
Can you help me to learn more?
(Other definitions for effort that I've seen before include "Struggle" , "Toil" , "The use of physical or mental energy" , "Strenuous endeavour" , "Determined attempt" .)